Flexible circuits, or "flex" circuits, are commonly used to interconnect electronic devices, and particularly electronic devices that exist on printed circuit boards. Flexible circuits have the ability to bend so as to exist in three dimensions, rather than the two dimensions within which printed circuit boards lie, and, therefore, are a particularly advantageous way of interconnecting printed circuit boards that lie in different planes or are subject to relative movement. Although flex circuits provide an advantageous means for interconnecting electronic devices existing on rigid printed circuit boards, the connection of the flex circuit to the printed circuit board is of critical importance if it is to operate properly.
A flexible circuit is typically connected to a printed circuit board through multiple finger-like projections that extend from one end of the flex circuit. These "fingers" are exposed copper connections that are extensions of traces in the flex circuit. Each finger will correspond to a pad that is etched on the printed circuit board to which it is to be connected, with each pad being an extension of a trace of the printed circuit board. The corresponding fingers and pads are electrically and mechanically coupled together by solder, with each separate corresponding finger-pad pair being individually soldered together. In a typical flex circuit there are many fingers positioned in close proximity to one another. The center-to-center distance between adjacent fingers can be in the order of 15-20 thousandths of an inch and will likely grow smaller in the future.
Known methods for soldering a flex circuit to a printed circuit board involve what is known as "hot bar" reflow process. Solder is placed on each finger of the flex circuit and on each corresponding pad of the printed circuit board in a process known as "tinning." The flex circuit is then positioned so that the solder on the corresponding fingers and pads are opposite and adjacent to one another. A hot bar is then brought down onto the printed circuit board so that it is physically touching and applying pressure to the printed circuit board across a location just opposite the pads of the printed circuit board. The hot bar is sized to match the footprint of the area to be soldered. The heat from the hot bar is conducted through the printed circuit board and through the pad and solder, causing the solder to reflow to form a solder joint between the printed circuit board and the flex circuit.
The hot bar reflow process has problems and disadvantages associated with it that render it difficult to make reliable, consistent solder joints across all fingers and corresponding pads. First, temperature variations across the bar are common, resulting in variations in the quality of the solder joints. Similarly, any variation in the way in which the bar is positioned against the printed circuit board will affect the quality of the solder joints. For example, if the hot bar is not perfectly parallel, heat will be applied unevenly across the solder area. More importantly, the pressure that must be applied by the bar against the printed circuit board will often result in the solder being squeezed out of the target area where the connection is to be made. This can result in adjacent fingers and pads being interconnected, creating a defect known as "solder bridging," which can have detrimental affects on the operation of the assembly. The hot bar reflow process can also result in weak solder joints. Moreover, the quality of the solder connection is difficult to determine due to the lack of a peripheral meniscus around the joined pads.
The hot bar reflow process can also cause physical destruction or degradation of the printed circuit board itself. The temperature and pressure of a hot bar that is brought in direct physical contact with the printed circuit board frequently causes burning or scorching of the surface of the printed circuit board and sometimes will also cause warping or delamination. Further, the plastic or polymer substrate material of the printed circuit board and the copper traces of the printed circuit board have significantly different rates of thermal expansion. As the polymer substrate expands more rapidly under the influence of the heated hot bar, the copper traces tend to pull away from the polymer substrate, causing the copper fingers to become delaminated from the printed circuit board.
Another method by which soldering can be done is through point-to-point connections, rather than by attempting to form all solder joints at once, as with a hot bar solder reflow process. The point-to-point method is extremely labor intensive and unreliable due to the limit of human ability to work accurately with such close pitches.